In the drilling, completing of oil wells, it is often necessary to isolate particular zones within the well. In some applications, downhole tools, known as bridge plugs, fracture (“frac”) plugs, and the like, are inserted into the well to isolate zones. The purpose of the bridge plug or frac plug is to isolate some portion of the well from another portion of the well. For example, perforation in the well in one portion may need to be isolated from perforations in another portion of the well, or there may be a need to isolate the bottom of the well from the wellhead. Accordingly, the plug may experience a high differential pressure, and must be capable of withstanding the pressure so that the plug seals the well and does not move in the well after being set.
Because downhole tools are used in a wide range of well bore environments, they must be able to withstand extremes of high temperature and pressure. During normal well completion operation, the downhole tools must be removed to allow the installation of tubing to the bottom of the well to begin the recovery of oil or gas.
A plug is generally comprised of one or two slips and cones as well as an elastomeric packing element arranged about a mandrel that is run into the wellbore. The slip may be initially formed in a ring, and designed to break apart upon the application of an axial force. The slip includes a tapered surface that is adapted to mate with a tapered surface of the cone. As an axial force is applied to the plug, relative movement between the slip and the cone happens, the slip moves up on the tapered surface of the cone and breaks apart to form a number of individual slip elements, and the slip elements are driven outwardly, away from the mandrel, and thus engages the casing wall, locking the slip in place within the casing.
Further application of axial force compresses the elastomeric packing element, driving the packing element outwardly to contact and seal against the wellbore. The axial compression of the packing element causes the packing element to expand radially against the well casing creating a sealing barrier that isolate a portion of the well.
Unfortunately, it has been found that once the plug is set, if the slip is not centered within the wellbore the slip elements may not be uniformly disposed around the inside walls of the casing. This non-uniform position of the slip elements results in uneven stress distribution around the mandrel. An uneven stress distribution may limit the axial load capacity of the slip and may result in movement of the plug over time as it is used in the casing, which results in a loss of seal or damage to other well components.
When it is desired to remove one or more of these plugs from a wellbore, it is often simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the plug. In drilling, a drilling bit is used to cut and grind up the components of the plug to remove it from the wellbore. Problems sometimes occur during the milling or drilling of the plug. For instance, the plug components can bind upon the drilling bit and rotate with it within the casing. Such binding can result in extremely long drill-out times, excessive casing wear, or both. Long drill-out times are highly undesirable, as rig time is typically charged by the hour.
Disadvantages with prior art plugs can be excessive length of the plugs since all of the above combined systems require length. It would advantageous to have a short plug, so if plug removal is required, milling time would be greatly reduced.
In light of the foregoing, there exist a need for a plug that forces the slip into the casing wall to distribute the load more uniformly when set into the wellbore, thereby improving the axial load capacity of the slip. Further, a plug that can be easily, quickly, and reliably removed from the wellbore may also be desirable.